1. Field of the Invention
The present invention relates to an optical pickup which can remove birefringence due to the non-uniformity of an optical recording medium. More particularly, the present invention relates to an optical pickup which can remove birefringence due to the non-uniformity of an optical recording medium by using a polarization beam splitter.
2. Description of the Related Art
In general, an optical pickup records information on an optical recording medium and/or reproduces information therefrom in a non-contact manner using light emitted from a light source.
Referring to FIG. 1, a conventional optical pickup includes a light source 1 emitting light, a beam splitter 5 changing a proceeding direction of incident light, an objective lens 11 arranged on an optical path between the beam splitter 5 and the optical recording medium 13 to converge the incident light on an optical recording medium 13, a collimating lens 7 arranged on the optical path between the beam splitter 5 and the objective lens 11 to converge incident divergent light to a parallel beam, and a photodetector 17 for receiving light reflected from the optical recording medium 13 and detecting an information signal and error signals.
The beam splitter 5 is a polarization beam splitter which reflects one polarized light and transmits the other polarized light. That is, in the optical arrangement shown in FIG. 1, the light emitted from the light source 1 is reflected to proceed toward the optical recording medium 13, while the light reflected from the optical recording medium 13 is allowed to pass through the beam splitter 5 and proceed toward the photodetector 17.
A quarter wave plate 9 is provided on the optical path between the beam splitter 5 and the objective lens 11 to make a circularly polarized beam incident on the optical recording medium 13. The quarter wave plate 9 converts an incident linearly polarized beam into a circularly polarized beam, and the circularly polarized beam into the linearly polarized beam.
The photodetector 17 includes a plurality of section plates that independently receive light and perform photoelectric conversions, and which further detects an information signal recorded on the optical recording medium 13, a track error signal, and a focus error signal by selectively differential amplifying and summing signals at each of the section plates.
A three beam method is widely used to detect the track error signal. For this use, a grating 3 is provided on the optical path between the light source 1 and the beam splitter 5 to diffract and transmit incident light as the 0th order light, ±1st order light, ±2nd order light, and so on.
An astigmatism method is also widely used to detect the focus error signal. For this use, an astigmatism lens 15 is provided on the optical path between the beam splitter 5 and the photodetector 17. The astigmatism lens 15 is inclined in a direction that is reverse to a direction in which the beam splitter 5 is inclined to correct coma aberration.
The optical recording medium 13 is a disc type optical recording medium such as a compact disc (CD) or a digital versatile disc (DVD). The optical recording medium 13 is manufactured by pressing resin injected in a mold so that, in some cases, the resin may be formed non-uniformly in a space in the mold. The density of the optical recording medium that varies according to the degree of non-uniformity. Thus, when information is recorded/reproduced with respect to portions of the optical recording medium having different densities, a refracting angle of the incident light varies as if passing through media having different refractive indices. Thus, the non-uniformity of a medium causes a birefringence phenomenon.
When light is incident on a portion of the optical recording medium 13 where the density thereof satisfies a desired standard, and on a portion of the optical recording medium 13 where the density thereof does not satisfy the desired standard, different refraction phenomenon occurs. Thus, the refracting angle becomes different and can be divided into light having a desired polarization and light having a different polarization. The amount of polarization is dependent on a degree of the birefringence.
The reflection and transmission properties of the beam splitter 5 are set to satisfy the polarization property of a beam emitted from the light source 1 so as to transmit and allow only light having a normal polarization to proceed toward the photodetector 17. That is, light having an abnormal polarization does not proceed on to the photodetector 17. Thus, the amount of light decreases at a portion of the optical recording medium 13 where the density thereof varies.
The optical recording medium 13 is rotated by being installed on a spindle motor (not shown). Thus, recording/reproducing of information is periodically performed with respect to the portion where the density varies so that the increase and decrease in the size of a signal received through the photodetector shows periodicity. Such periodicity lowers reproduction performance and works as an error in spite of signal reproduction and signal change.
According, a need exists for a system and method to remove or minimize the undesired effects due to the non-uniformity of an optical recording medium.